The present invention relates generally to vehicle suspension systems that employ leaf springs, and more particularly to leaf springs incorporating layers of composite material and methods for fabricating said springs.
Known leaf springs are constructed from several elongated strips or leaves of metal stacked one-on-top-of-the-other and clamped together in a substantially parallel relationship. Typically, these springs are employed in vehicle suspension systems in one of two different load carrying configurations, cantilevered, or three-point-bending; the latter being the more common method of use. A cantilevered leaf spring is one where the leaf spring is fixed or supported at one end to the frame of a vehicle and coupled to an axle at its other end. Alternatively, a leaf spring mounted in three-point-bending, is supported or fixed at one end to the vehicle with the other end supported in a manner that allows for relative movement of the spring. A load is carried by the spring between the two ends. The use of leaf springs mounted in three point bending is so widespread that the Society of Automotive Engineers (SAE) has developed a formal leaf spring design and use procedure.
Metal leaf springs constructed in the manner described above are incorporated into a variety of different vehicle suspensions including, automobiles, light to heavy trucks, trailers, construction equipment, locomotives, and railroad cars. They are also employed in recreational vehicles, such as bicycles, snowmobiles, and ATV""s (all terrain vehicles). The leaf springs improve the quality or smoothness of the vehicle""s ride by absorbing and storing energy for later release in response to bending and/or impact loads imposed on the vehicle resulting from such things as encountering obstructions in a road during the vehicle""s operation.
The mechanical properties defining a vehicle suspension system, particularly the spring rate and static deflection of the leaf springs, directly influence the smoothness of the vehicle""s ride. Generally, a smooth ride requires the leaf springs to have large static deflections. The smoothness of the ride is also influenced by the vibration damping characteristics of the leaf springs. Damping is a parameter that quantifies the ability of the leaf spring to dissipate vibratory energy. Therefore, a high degree of damping is desirable in leaf springs used in automobiles to minimize the vibratory amplitudes transferred to the passenger area.
The ability to accurately determine the mechanical properties and performance characteristics of a leaf spring is critical to the proper design of vehicle suspension systems. One of the problems resulting from the construction of conventional leaf springs is that the variable lengths of the stack of individual leaves creates a stepped spring construction that only approximates constant stress, the steps tend to create localized areas of high stress known as stress concentrations which detrimentally affect the load carrying capability and useful life of the leaf spring. In addition, the fact that the springs are composed of lengths of metal stacked one-on-top-of-the-other causes them to be quite heavy; this additional weight causes a concomitant reduction in fuel economy.
Moreover, because it is impossible to predict the exact conditions and stresses that a leaf spring will be subjected to, the fatigue life of the spring is generally limited. This problem is further exacerbated by the build-up of foreign material on and between the individual leaves. Not only does this cause corrosion, thereby weakening the leaf spring and making it more susceptible to fatigue failure, but it also affects the stiffness of the leaf spring and hence the smoothness of the ride of the vehicle in which the spring is employed. Generally the magnitude of the contribution made to the strength of a particular leaf spring due to inter-leaf friction is determined empirically. When foreign material gets between the leaves it can dramatically increase, in the case of particulate matter, or decrease, in the case of oil, the friction between the leaves, thereby altering the original mechanical properties of the spring. In addition, the shear conductivity between the leaves, which is a measure of the amount of shear stress transferred from leaf-to-leaf, is typically low in conventional leaf springs because the individual leaves are only clamped at the ends. Therefore, the stress transfer capability along the length of the spring is dependent on the aforementioned inter-leaf friction.
In many applications, leaf springs are loaded not only by vertical forces but also by horizontal forces and torques in the longitudinal vertical and transverse vertical planes. These forces are typically generated when the brakes on the vehicle incorporating the leaf spring are applied. The aforementioned horizontal forces and torques cause the leaf spring to assume an xe2x80x9cSxe2x80x9d shaped configuration, a phenomena referred to as xe2x80x9cS-ingxe2x80x9d or wrap-up. The stresses induced in the spring when this occurs can be quite high. In order to minimize S-ing in a leaf spring, the stiffness of the spring must be increased; however, this can detrimentally affect the smoothness of a vehicle""s ride.
In order to address the above-described problems, those skilled in the art have attempted to fabricate purely composite leaf springs, wherein the individual leaves are formed from a composite material of the type consisting of a plurality of fibers embedded in a polymeric matrix. However, while these springs offered significant reductions in weight, as well as increased fatigue life and damping, their cost was prohibitive. In addition, these composite springs are difficult to fabricate and attach to the frame of a vehicle and required the use of special adapters. A hybrid leaf spring having a metal primary leaf with one or more layers of composite material bonded thereto has been proposed in U.S. patent application Ser. No. 08/906,747 to Meatto, Pilpel, Gordon and Gordon entitled xe2x80x9cHybrid Leaf Spring And Suspension System For Supporting An Axle On A Vehiclexe2x80x9d, filed on Aug. 6, 1997, the disclosure of which is incorporated herein by reference. The metal primary leaf also defined the means, for example, an aperture extending through each end of the leaf, to mount the spring to the vehicle.
Composite components usually comprise multiple individual layers of material juxtaposed, one on top of the other with adhesive material located between successive layers of the composite, thereby forming a laminate. As used herein, the term xe2x80x9ccomposite materialxe2x80x9d should be construed to mean a fiber or particle reinforced polymeric material. To bond the layers of composite material together, the adhesive must be cured unless a thermoplastic adhesive is used which requires only melting and fusing. Curing is usually accomplished by heating the composite layers under pressure in a mold to a known curing temperature and then maintaining that temperature for a predetermined period of time.
A difficulty often encountered with producing laminated composite components in this manner is that the individual layers of composite material act as insulators. Therefore, to completely cure a multiple layer laminated composite part, long heating periods are required to allow the adhesive between the inner-most layers to reach curing temperature. This results in decreased productivity, increased energy consumption, wear on the mold, and higher overall cost. These problems are further exacerbated with respect to the above-described hybrid leaf spring because the metal primary leaf acts as a heat sink, drawing thermal energy away from the adhesive material.
Another difficulty encountered with producing hybrid leaf springs is that for applications with high spring rate camber designs, molded-in bond line shear stress between the primary leaf and composite layers can be relatively high so as to reduce fatigue life when the hybrid leaf spring is fully deflected under a full load.
Based on the foregoing, it is the general object of the present invention to provide a leaf spring and a method for fabricating the spring that overcomes the difficulties and drawbacks of prior art leaf springs.
The present invention is directed in one aspect to a method for making a hybrid leaf spring wherein at least one layer of composite material, and at least one metal primary leaf are laminated together. To facilitate the lamination process, the layer of composite material and the primary leaf are positioned adjacent to one another in an interior cavity defined by a mold. A layer of adhesive is located between and in engagement with the layer of composite material and the metal primary leaf.
Heating means are coupled to the metal primary leaf and are actuated via command signals generated by a controller having temperature profile data stored therein. During operation, the heating means imparts thermal energy to the metal primary leaf which in turn is transferred to, and cures the adhesive material.
Preferably, the heating means is of the resistance type with the metal primary leaf forming part of the heating circuit. In general, a voltage source is provided that includes at least two electrodes attached thereto. Each electrode is also releasably attached to an end of the metal primary leaf, thereby completing the circuit. The primary leaf defines an inherent resistance such that when the voltage source is actuated, the current flowing through the primary leaf, between the electrodes, causes the temperature of the primary leaf to increase. This thermal energy is then transferred from the primary leaf into the layer of adhesive material. The current is varied in response to command signals issued from the controller in order to create the appropriate temperature profile to allow the adhesive material to cure. While a resistance-type heating means has been described, the present invention is not limited in this regard as other types of heating means, such as, but not limited to an induction heater, or a convection-type heater can be substituted without departing from the broader aspects of the present invention.
In the preferred embodiment of the present invention, the above-described mold is constructed of a material referred to by those skilled in the art to which the invention pertains as xe2x80x9ctooling boardxe2x80x9d. This material is typically formed from epoxy or polyurethane with fillers, such as ceramics. The tooling board has low electrical conductivity, thereby reducing the potential for arcing that could result from the resistance-type heating described above. While a mold made from tooling board has been described, the present invention is not limited in this regard as other materials, such as, but not limited to metal, may be substituted without departing from the broader aspects of the present invention. Where the mold is metallic, heating means, such as cartridge heaters, or passages for hot oil can be incorporated into the mold to supply additional thermal energy to the hybrid leaf spring during curing of the adhesive.
In an embodiment of the present invention, the above-described at least one layer of composite material includes a plurality of layers of composite material. Each layer is positioned in the mold adjacent to, and approximately aligned with, the next successive layer of composite material with at least one of the layers being adjacent to the metal primary leaf. A layer of adhesive, curable in the above-described manner, is positioned between successive layers of the composite material, as well as between the metal primary leaf and any adjacent layers of composite material. The adhesive is then cured via a combination of heat and pressure.
Alternatively, a layer of elastomeric material is interposed between the composite layers, as well as between the metal primary leaf and any adjacent layers of composite material. A layer of adhesive is spread between the elastomeric material and the composite layers, as well as between the metal primary leaf and any adjacent layers of composite material. In order to prepare the metal primary leaf, the elastomeric material, and the layers of composite material to accept the adhesive, a surface preparation step is usually required. For the metal primary leaf, surface preparation can be accomplished via sandblasting, vapor blasting, or chemical etching, with sandblasting providing the added benefit of slag removal from the metal. Regarding the composite layers, surface preparation is usually achieved via sanding or diamond grinding. The elastomeric layers can be surface treated by, inter alia, etching or embossing. Thermoset type elastomers can also be sanded or ground, while thermoplastic material can be flame treated, corona discharge treated and inert plasma treated. In some instances, the above-described treatments can be combined with sanding and grinding.
Depending on the end use of the hybrid leaf spring made in accordance with the present method, it may be necessary to coat all or part of the spring with a protective coating to increase impact resistance. Alternatively, it may be necessary to coat only those areas where an adhesive layer is exposed to the outside environment.
In one embodiment of the hybrid leaf spring fabricated in accordance with the method of the present invention, precured composite layers are employed with at least one face of each layer being machined or ground to provide the desired contour of the finished spring. When the machined layers of composite material are placed in the mold, the machined or ground face of one leaf becomes the bonding surface between that leaf and the metal primary leaf and is positioned adjacent to the metal primary leaf with a layer of adhesive therebetween. The precured composite layers, and the metal primary leaf can be assembled inside, or outside of the mold with a pin locating the components relative to one another. The pin can also be employed to aid in positioning the uncured spring to the mold.
In another embodiment of a method of fabricating a hybrid leaf spring in accordance with the present invention is employed for applications with high spring rate camber designs. For such applications, molded-in bond line shear stress can be relatively high and reduce fatigue life when the hybrid leaf spring is fully deflected under a full load. In accordance with the method of the present invention, a primary leaf is provided having a free height or initial camber, and at least one layer of composite material. A mold is provided having an interior cavity defined by at least one cavity wall. The interior cavity has a curvature for receiving the primary leaf therein at a depressed camber relative to that of the initial camber in order to pre-stress the primary leaf. The layer of composite material and the primary leaf are positioned adjacent to one another in the interior cavity. A layer of adhesive is positioned between and in engagement with the layer of composite material and the primary leaf. The layer of adhesive is cured for bonding together the primary leaf and the layer of composite material at the depressed camber to thereby reduce bond line shear stress. More specifically, the adhesive is cured at a depressed camber at or near the curb height of the spring so as to generally eliminate bond line shear stress when the camber of the hybrid leaf spring is at the curb height (i.e., the camber of the installed hybrid leaf spring when not absorbing loads).
The depressed camber of the hybrid leaf spring in the mold is between an unloaded camber and a curb load (i.e., full load) camber. The hybrid leaf spring is removed from the mold such that the camber of the removed hybrid leaf spring increases to a camber between that of the initial camber of the primary leaf and the depressed camber of the hybrid leaf spring in the mold.